Compared to other primates, even larger primates such as the gorilla, the male human genitalia are remarkably large. The human penis is both longer and thicker than that of any other primate both in absolute terms and in relative size compared to the rest of the body.
Measuring the penis
There are different ways to measure a penis, and some difficulties to overcome while doing so. First of all, a penis must be maximally erect before measurement, and in a clinical setting this is difficult to achieve. At least one Brazilian doctor resorted to injecting penises with drugs to induce an erection, giving much more consistent results. Some clinicians measure the penis by stretching the flaccid penis as far as comfortably possible and take the measurement based on the theory that a fully stretched flaccid penis is equal to an erect penis length. Relying on self-reporting of penis size is problematic, since some patients exaggerate or are unable or unwilling to measure the penis correctly.
Penis length is typically measured with the subject standing and the penis held parallel to the floor. Measurement of length goes horizontally along the top of the penis from the penis's base to the tip. If the penis is measured along the bottom or underside, the result is less reliable. Measurements taken while sitting or lying down also tend to be unreliable.
Girth is measured by using a tape measure. It is variously quoted as an average, at 3 places along the penis, just below the penile head, in the middle of the shaft, and at the base or the thickest part.
There have been several studies regarding the average size of the human penis. The majority of such studies could be unreliable due to self-selection bias: men with a smaller than average penis might be less likely to allow themselves to be measured, while men with a larger than average penis might be more likely to allow themselves to be measured. It is unknown if this conjectured self-selection bias has been confirmed by any studies, or if any studies have attempted to account for potential self-selection bias regarding penis size.
Size at birth
Average stretched penile length at birth is about 4 cm (1.6 in), and 90% of newborn boys will be between 2.4 and 5.5 cm (0.9 and 2.2 in). Limited growth of the penis occurs between birth and 5 years of age, but very little occurs between 5 years and the onset of puberty. The average size at the beginning of puberty is 6 cm (2.4 in) with adult size reached about 5 years later. W.A. Schonfeld published a penis growth curve in 1943.
Regarding the length of the adult fully erect penis (measured along the top of the penis from the groin to the tip), several studies have been performed. Studies that have relied on self-measurement, including those from Internet surveys, consistently reported a higher average than those that used medical or scientific researchers to perform the measurements.
Academic and scientific studies
- A study published in the September 1996 Journal of Urology concluded that average erect length was 12.9 cm (5.08 in)(Measured by staff).
- A study published in the December 2000 International Journal of Impotence Research found that average erect penis length was 13.6 cm (5.35 in)(Measured by staff).
Sample of college students on spring break
- A study conducted by LifeStyles Condoms during 2001 Spring Break in Cancún, Mexico, found an average of 14.9 cm (5.9 in) with a standard deviation of 2.1 cm (0.8 in)(Measured by staff).
Self measurement surveys
- An Internet survey performed by Sizesurvey.com found an average of 16 cm (6.3 in).
- An Internet survey performed by Jackinworld.com found an average of 15.6 cm (6.1 in).
- An Internet survey performed by HowLongIsYours.com found an average of 15.8 cm (6.2 in).
Similar results exist regarding studies of the circumference of the adult fully erect penis (with the measurement taken mid-shaft). Just as with length, those studies that relied on self-measurement consistently reported a higher average than those that had staff take the measurements, possibly indicating exaggeration when the penis is not scientifically measured.
- A study conducted by LifeStyles Condoms during 2001 Spring Break in Cancún found an average of 12.6 cm (5.0 in) with a standard deviation of 1.3 cm (0.5 in).
Length of the unencumbered flaccid penis is not necessarily proportional to size of the erect penis. Blood flow from arousal, and its relationship to the subsequent length measurement of the erect penis varies widely among men. Hence a smaller-than-average flaccid penis might grow to five times its flaccid length and twice its girth, or conversely a flaccid penis on the larger side might only grow to 1.5 times the length when filled with blood. The transition to erection and its relationship to measurements is by no means a one-size-fits-all equation.[Citation needed] The slang term for this disparity is referred to as a penis being a "grower or a shower," the former being flaccid penis that becomes proportionally larger when erect, the latter being a penis that grows very little in the transition between flaccid and erect.
- A study published in the September 1996 Journal of Urology concluded that mean flaccid length was 8.8 cm (3.5 in)(Measured by staff).
Low temperatures affect the size of the flaccid penis. One general physiological response to cold is decreased circulation of blood to the extremities, which includes the external genitals. The penis will often shrink further as a result, in the process known as shrinkage.
In a 2005 study by the University of California, Los Angeles, 45% of men responded they would prefer their penis size increased. 84% of respondents rated their penis size as average to above average. (There is a similar perception pattern in women's perceptions of their breasts.)
A study undertaken at Utrecht University found that the majority of homosexual men surveyed regard a large penis as ideal, and having one is linked to self esteem.
Men may tend to misjudge the size of their penis relative to that of other men they have seen naked, simply because of the foreshortening effect obtained from always looking down at the penis from above. In addition, as Paul Fussell noted in his memoirs, men who are overweight or have large stomachs may fail to allow for the partial concealment of the penis by their abdomen. The accumulation of fat on the pubic bone above the penis may give a shorter appearance even though the length of the penis from the base is normal.
A survey by sexologists showed that many men who believed that their penis was of inadequate size actually had an average-sized penis. Most sexologists believe that worries about penis size come from some other sources of anxiety or perceived inadequacy.
In ancient Greek art, it is common to see smaller male genitalia than one would expect for the size of the man. Renaissance art also followed this aesthetic; note Michelangelo's David. This was due to the belief that an uncircumcised and small penis was desirable in a man, whereas a bigger or circumcised penis was viewed as comical. This may have had to do with the Greeks' adoration of an undeveloped body or the actual size of real models who stood in cold studios. However, it has also been said that artists intentionally depicted the penis size as smaller than usual, to prevent it from detracting from the art form itself. Others have said that perhaps male average penis size back then was significantly smaller, and has become bigger over time much like the average height has. It is also believed that a large penis was believed to be "beastly" or animal. 
Legends & Pop Culture
In some cultures, mass hysteria involving the believed removal or shrinking of the penis has been observed (genital retraction syndrome).
Many beliefs in popular culture hold that it is possible to predict the size of someone's penis by observing other features. The features usually selected are the hands, the feet, the nose or height. One urban legend suggests that the size of a fully erect penis is the length from the tip of a man's thumb to the tip of his index finger, when a 90° angle is made with these two digits. (An alternative formulation is that this indicates the flaccid penis's length.)
The suggested link between penis size, foot size and height has been investigated by a relatively small number of groups. Two of these studies have suggested a link between penis size and foot size, while the most recent report dismissed these findings.[Citation needed] One of the studies suggesting a link relied on the subjects measuring the size of their own penis, which may well be inaccurate. The second study found statistically significant although "weak" correlation between the size of the stretched penis and foot size and height.
Recently, there has been greater media attention to the issue of penis size and women being more vocal about their preferences. Television shows such as Sex and the City and Ally McBeal popularized the penis-size issue when characters in these TV shows stated their preference for well-endowed men and rejected men who had only average endowment. In HBO's Sex and the City, a female character is portrayed crying in bed when she experiences her boyfriend's penis for the first time and discovers that it is below average length, as opposed to the large length she had expected and anticipated.
The media have been criticized for making "penis envy" into a male body issue equivalent to Cosmopolitan magazine being criticized for their coverage of women's weight. Indeed, one episode of Sex and the City also shows a character expressing displeasure over her partner having too large a penis, though the penis is described in hyperbolic terms and implied to be something of impossibly gargantuan proportions. The British television series Coupling features a running joke on the huge penis of one of the males and the fascination women have with it.
Surveys of women's actual preference have consistently shown that penis size is only a priority for a minority of women, and some women dislike large penises.
One source of continued debate is the extent to which heterosexual and bisexual women actually prefer certain penis sizes. In a 2005 UCLA study, 85% of women said they were "very satisfied" with their romantic partner's size.
A study published in BMC Women's Health, surveyed women's preferences concerning penis size and concluded that width rather than length is a more important factor of sexual stimulation.
Another study, conducted at Groningen University Hospital, asked 375 sexually active women (who had recently given birth) the importance of penis size and concluded: "Although clearly in the minority, a nevertheless considerable percentage of the women respondents attached substantial importance to the size of the male sexual organ".
In essence, it is believed that a man with a large penis is more desirable for the average woman with an average vaginal cavity.
Penis size and vaginal response
The vagina itself is a very elastic environment which can expand on average to ~10 centimetres (4 in) in diameter during childbirth, and it also easily retains a tampon. Thus a woman's vagina can tightly accommodate most penises from relatively small to large. It will generally accommodate and adjust to the object it surrounds. Discomfort can sometimes be experienced with unusually large objects, but the vagina can, over time, adapt.
According to Louanne Cole Weston, PhD, licensed counselor, and sex therapist, in a May 2002 report by WebMD, several misconceptions have developed surrounding penile-vaginal intercourse. Cultural preferences may have exaggerated the importance of deep vaginal penetration in obtaining female orgasm.
The most sensitive area of the vagina is the section closest to the outside of a woman's body, which is roughly 10 centimeters (4 in) in length. Given that the average penis size is above this length, most men should be able to easily reach and stimulate these erotic nerve endings.
In stark contrast, minor surgery without anesthetic can be conducted on the inner portion of a woman's vagina without discomfort. Most women attest to a feeling of being "filled up" by larger than average penises, yet few can claim to feel erotic sensations in the deeper regions of the vagina.[Citation needed] In fact, stimulation of the G-spot may be more effective if the man's penis is slightly shorter than average, as this highly sensitive area of the vagina is located closer to the opening of vagina than to the recesses of its canal (found anywhere from 1 to 3 inches inside the vagina, on the anterior wall).
Some women have reported enjoying stimulation of the cervix, although this may be confused with the anterior fornix (deepest point of vagina above cervix) or posterior fornix (back of vagina, behind cervix). The cervix, fornix, and cul-de-sac are within close proximity of each other, so there may be indirect and/or simultaneous stimulation between them.
The fornix is said to be another possible orgasm trigger area. Tests have shown that pressure on this area causes the vagina to lubricate very quickly. The area of sexual response in the anterior fornix has also been called the epicentre, T-Spot, AFE-Zone, AFE or A-Spot; while in the posterior fornix it has been called epicenter (as well) or cul-de-sac (since the cul-de-sac may be indirectly stimulated by pressure on the posterior fornix).
No matter the size of a man's penis it takes a level of skill to stimulate the anterior fornix. The man with a shorter penis may also be able to stimulate the anterior fornix using some advantageous positions which shorten the distance to the uterus.
Long penises (over ~20 centimeters (8 in)) can be susceptible to rubbing on, or striking, the cervix, which has been described as being rather uncomfortable, or sometimes painful. Over-stretching the back of the vagina can also cause discomfort, and this may be mistaken for cervix pain. Thus very long penises sometimes cannot be fully inserted. The exact depth depends on both anatomical dimensions and arousal of the woman, as well as the sexual technique used.
During sexual intercourse, the vagina lengthens rapidly after initial insertion from about 10 to 14 cm (4 to 5.5 in), but the final and initial depths vary from woman to woman ±2.5 cm (±1 in). The vagina (with proper arousal) will stretch to envelop larger sizes (up to a point) — about 19–23 cm (7.5–9 in) maximum on average (although more may be accommodated with long-term adaptation). As the woman becomes fully aroused the vagina tents (last ²⁄₃ expands in length and width) while the cervix retracts, meaning that with the right penetration angle long penises will slide over or under the cervix without directly hitting it (instead stretching the back of the vagina or anterior/posterior fornix). Enough foreplay to produce full arousal is generally needed to ready a woman for penetration by a large penis — although this same amount of foreplay is often recommended regardless of the man's penis size. While a long penis normally isn't a hindrance, there's little to suggest it's advantageous either.
On the other hand, stimulation of the G-Spot may be more effective if the man's penis is thicker than average, since the pleasure sensations from this area are activated primarily by pressure. A thicker penis may provide more friction against the vestibular bulbs, which are located in close proximity and anteriorly on either side of the urethra.
Women have confirmed in surveys the primary focus of the clitoris in sexual stimulation. Roughly three-quarters of women surveyed have reported difficulty reaching orgasm by vaginal intercourse alone, and just 7% are able to do this on most occasions. Many report requiring simultaneous clitoral and vaginal stimulation — regardless of the dimensions of the inserted object.
Penis size and condom use
Ninety-two monogamous heterosexual couples (aged 18 to 40 for women, 18-50 for men) were enrolled in a prospective study of Durex Ramses condoms. At each sexual encounter, a diary was completed which included information on condom use, and breaks and slips.
In France a random telephone survey of 20,000 individuals drew on 4,500 sexually active people, of whom 731 had used a condom in the previous year and 707 provided information on difficulties of use.
In Australia 3658 condoms were used by 184 men in a study which looked, inter alia, at penis size as a factor for breakage or slippage.
The rate of breakage was 1.34% and of slippage 2.05%, with a total failure rate of 3.39%. Penis size was not related to slippage, but penis circumference was strongly associated with breakage.
- Lifestyle Condoms Penis Size Survey
- Kinsey Institute An information page on the penis.
- Penis Size Based on Body Parts
- Findings in a Hong Kong Study of Flaccid Penises
- Jackinworld.com Size Survey
- Jackinworld.com Size by Age
- Patient Queries - Penis Size Queries related to penis size answered by Sexologists & Psychiatrists.
- Documentary of Penis Pictures and Surveys Devoted to proving the penis is more than a size.
- HowLongIsYours.com Online penis size survey and results
- Durex Penis Size Survey